Randy Hines, age 11, of High Point, No. Carolina, for his question:
What causes natural gas?
Natural gas is formed by natural processes in the earth and all the evidence indicates that the same recipe is used to create oily petroleum. The process has been repeated time and again. Some samples are found in rocks that date back half a million years ago. A few pockets of natural gas are forming right now.
During the past 500 million years or so, enormous quantities of natural gas and petroleum have been formed in the earth's crust. Nobody can estimate how much of these valuable fuels have evaporated and been lost to us. The remaining deposits sur¬vived because they are trapped in special rock formations and sealed underground. The ingredients in these natural fuels suggest how they originated. The geological forma¬tions around the deposits suggest how they were processed.
The basic ingredients in both natural gas and petroleum are hydrocarbons. Hence, they are related to the biochemicals produced by the living cells of all plants and animals. They are called organic chemicals because their carbon content associates them with the processes of life. For this reason, most geologists favor the idea that both oil and natural gas are fossil materials. A few diehards insist that they could have been made from inorganic chemicals. But a fossil origin appears by far the most logical explanation.
Natural gas contains methane and ethane, propane and butane. These light molecules of hydrogen and carbon also are present in petroleum, blended with a more complex assortment of other hydrocarbons. Most likely these basic ingredients were manufactured by living organisms that thrived in shallow seas. Sunlit surface waters have teemed with plant and animal populations since life on earth began.
As the marine organisms lived and died, the water became a chemical broth enriched with decomposing hydrocarbons. These oily residues are lighter than water, though possibly they found a way to sink and mix with the soggy mud. Meantime in many areas, the restless earth lifted ancient sea beds and covered them with new layers of dry land.
In many regions, vast deposits of marine residues were trapped underground. Many were so near the surface that the volatile hydrocarbons percolated through the rocky pores and escaped into the air. Other deposits were sealed below in rocky prisons through millions of years. This occurred when the organic residues saturated shale or other porous rocks, which were sealed below a dense roof of, say, granite.
Through the ages, this buried treasure was processed by heat and pressure. In some deposits, the fossil fuel became oily petroleum. In others, some of the lighter hydrocarbons rose up and formed pockets of natural gas on top of the petroleum. And in some regions, the ingredients formed vast deposits of only natural gas. There the gaseous hydrocarbons may be trapped in fractured underground formations and pockets of natural gas are found here and there at different levels.